1. Field of the Invention
This invention relates to the field of temperature sensing circuits, and particularly to circuits which force multiple emitter currents through a bipolar transistor to sense temperature.
2. Description of the Related Art
Bipolar transistors (BJTs) are frequently used as thermal sensing devices, since a BJT's base-emitter voltage (Vbe) varies with temperature in accordance with:Vbe=nFkT/q*ln(IC/IS)where nF is the BJT's emission coefficient, k is Boltzmann's constant, T is absolute temperature, q is the electron charge, IC is the collector current, and IS is the saturation current. For integrated circuits (ICs) fabricated using standard bulk CMOS processes, it is particularly convenient to use substrate PNP (SPNP) transistors to sense temperature. These SPNP devices can be located, for example, on a remote die (e.g., a CPU) which is intended to have its temperature measured by another circuit located on a separate die (e.g., an ASIC).
Methods of employing BJTs to sense temperature are described, for example, in U.S. Pat. Nos. 5,195,827, 5,982,221, and 6,097,239. These references, which employ one or more PNP transistors as thermal sensors, force two or more emitter currents which are in a fixed ratio (N) to each other (I, N*I) to create two ratioed collector currents (IC, ICN). When so doing, the above non-linear equation is simplified such that the temperature of the BJT is a linear function of absolute temperature (T). Assuming N=ICN/IC:VBEN−VBE1=ΔVBE=(nFkT/q)ln(ICN/IC), andVBEN−VBE1=ΔVBE=(nFkT/q)ln(N),where VBEN and VBE1 are the BJT's base-emitter voltages for emitter currents of N*I and I, respectively.
However, though the ratio of emitter currents may be fixed, the ratio of the resulting collector currents depends on the BJT's beta value (β)—which varies with collector current and temperature. Thus, the accuracy of the measured temperature using this method depends on the ratio of the emitter currents, and the β value of the BJT and its variation. Assuming that two currents (I and N*I) are forced into the emitter of a SPNP transistor:
for emitter current I, collector current IC=βI/(β+1);
for emitter current N*I, ICN=N*βNI/(βN+1).
If βN=β+Δβ=β(1±ε), and ε=Δβ/β, and assuming βN=β(1+ε):ΔVBE=(nFkT/q)[ln(ICN/IC)]  [Eq. 1a]andΔVBE=(nFkT/q){ln[[((1+ε)(β+1))/(1+ε)β+1)]*N]}, where ε=Δβ/β.  [Eq. 1b]From equation 1, it is clear that β errors affect the ratio of collector currents and therefore the measured ΔVBE voltage used to compute the device temperature. In addition, the accuracy of the temperature measurement may be reduced by ohmic resistances associated with the BJT, specifically its base and emitter resistances.
One method to solve equation 1a is to measure ICN and IC by simply subtracting the return base current from the forced emitter current, with IC=IE−IB and ICN=IEN−IBN. Once the two collector currents are measured, their ratios can be calculated. Another method is to force IE and measure IC=IE−IB. Then, force IEN until ICN=N*IC where ICN is measured as ICN=IEN−IBN. These methods are considered indirect methods, as the multiplied version of IC (i.e., ICN=N*IC) is obtained by forcing an emitter current and measuring the collector current indirectly as IC=IE−IB using a separate circuit.